1. Field of the Invention
The present invention relates to an apparatus for detecting a linear movement state or rotating state of an object and, more particularly, to a highly precise moving state detection apparatus utilizing the interference of light diffracted by a diffraction grating provided on a moving object.
2. Related Background Art
Photo-electric rotary encoders are conventionally used as a means for detecting a rotational speed and variations therein of rotating mechanisms in office equipment, such as a floppy-disk drive computer and a printer, an NC machine tool, the capstan motor of a VTR, a rotating drum, and the like.
A conventional photo-electric rotary encoder employs a so-called index scale system wherein a light projection means and a light-receiving means are arranged to oppose each other with a main scale and a stationary index scale sandwiched therebetween. The main scale is formed by arranging light-transmitting and light-shielding portions at equal angular intervals that the peripheral portion of a disk coupled to a rotating shaft. The index scale is formed by arranging the light-transmitting and light-shielding portions at the same angular intervals as in the main scale. According to this method, a signal in synchronism with the pitch of the adjacent light-transmitting and light-shielding portions of both scales can be obtained as the main scale is rotated. The obtained signal is subjected to frequency analysis to detect variations in the rotational speed of the rotating shaft. The smaller the pitches of the light-transmitting and light-shielding portions on both scales, the higher the detection precision. However, when the scale pitch is made to be small, the S/N ratio of the output signal from a light-receiving means is degraded by diffracted light, resulting in coarse detection. In order to prevent this, if a total number of light-transmitting and light-shielding portions of the main scale is fixed, thereby enlarging the intervals between the two portions to be increased to a point at which the light-receiving means is free from the effects of the diffracted light, the diameter and thickness of the disk of the main scale are increased, resulting in an increase in the overall apparatus size. Therefore, the object to be rotated is overloaded.
U.S. Pat. No. 3,726,595, and Japanese Patent Disclosure Nos. 190202/1982, 190203/1982, 207805/1982, and 98302/1985 disclose apparatuses each wherein a plurality of beams diffracted by a rotating diffraction grating are superposed to form interference fringes, and a displacement of the diffraction grating is measured by the change in contrast of the interference fringes. Such an apparatus utilizing interference of the diffracted beam enables higher precision measurement than that by the photoelectric encoder described above. However, in such an apparatus utilizing a plurality of diffracted beams, in order to form desired interference fringes on the photoelectrical detector, an optical system, which projects a coherent beam on the diffraction grating or superposes the beams diffracted by the diffraction grating on each other, must have a more precise arrangement, resulting in cumbersome assembly and adjustment.